MITOCHONDRIAL DNA VARIATION IN THE FUNGUS ATKINSONELLA HYPOXYLON INFECTING SYMPATRIC DANTHONIA GRASSES.

The purpose of this study was to determine how host identity and geographic proximity were related to genetic variation in the fungus Atkinsonella hypoxylon infecting four sympatric Danthonia grasses. We analyzed 192 A. hypoxylon isolates from 48 geographic sites for mitochondrial DNA (mtDNA) variation by restriction digestion of total DNA with EcoRI, BamHI, and PstI, and subsequent hybridization with purified A. hypoxylon mtDNA. Thirtynine haplotypes were identified. UPGMA cluster analysis showed that mtDNA type is highly correlated with host-species group; isolates from Danthonia compressa and D. spicata formed one branch of the phenogram, and isolates from D. epilis and D. sericea formed the other. Restriction maps of the most common mtDNA haplotypes infecting each host-species pair revealed a 30-kb size difference and a minimum of eight length changes and one restrictionsite change between them. Mapping of the mutation differences among all haplotypes occurring at North Carolina sites resulted in two distinct gene trees corresponding to the two Danthonia species groups, corroborating the phenetic analysis. The results indicate a high degree of host-dependent isolation and establish the existence of host races in A. hypoxylon. Possible mechanisms responsible for this isolation are discussed. Little differentiation existed between isolates from within a Danthonia species pair, and some variation was explained by geographic origin. Analysis of progeny from a natural sexual cross revealed that mitochondria are maternally inherited in A. hypoxylon.